This invention relates to a method and apparatus for warming high-pressure feed water heaters for use in starting thermal power plants.
The once-through boilers used for thermal power plants include Benson boilers and, in a certain kind of Benson boilers operated at a varying pressure, it is necessary to secure at the time of starting the boilers a predetermined circulation rate of feed water flowing therein. Accordingly, some of such boilers are provided with a so-called circulation system, wherein as shown in, for example, "Ishikawajima Harima Technical Report", Vol. 18, No. 2 (published in March 1978), FIG. 21, page 196, a part of the feed water is taken out from the portion of the boiler which is on the upstream side of a primary superheater, to be introduced into a liberating tank, the resulting feed water, from which vapor has been separated, being sent by a circulating pump to the portion of the boiler which is on the upstream side of a fuel economizer. In order to start a power plant provided with such a boiler, the boiler is ignited to circulate the feed water through a recirculation system therein. During such a period of time, that is, the time between an instant at which the boiler is ignited, and an instant at which the vapor starts being supplied to a steam turbine, the feed water in a deaerator provided in the portion of a feed water system which extends between a condenser and boiler is in a vacuum-deaerated state. According, the temperature of the feed water flowing through the first through third high-pressure feed water heaters, provided on the portion of the feed water system which extends between the deaerator and boiler, is increased to only about 60.degree. C. However, it is necessary that the temperatures of the feed water in the high-pressure feed water heaters be increased to about 130.degree.-190.degree. C. by practically utilizing these heaters in order, by the time the steam turbine has come to have about 20% partial load (by the time about twenty minutes have elapsed) after the vapor has started being supplied thereto. Therefore, during such a short period of time of about twenty minutes after the starting of supplying vapor to the steam turbine, the heated vapor is extracted from the turbine to be introduced into the feed water heaters, and the temperature of the feed water is thus increased suddenly from about 60.degree. C. to a target level of about 130.degree.-190.degree. C. However, when the feed water in the high-pressure feed water heaters is heated suddenly, the distribution of temperatures in the interior thereof becomes unbalanced due to the high variation rate of temperature. Consequently, a high thermal stress occurs in the heaters. This poses a problem of strength of the heaters.
An object of the present invention is to provide a method and an apparatus for warming high-pressure feed water heaters for power plants, which permits preventing the occurrence of an excessively high thermal stress in high-pressure feed water heaters when a power plant is started.
The present invention is directed to a method and an apparatus for warming high-pressure feed water heaters for power plants provided with a boiler having a circulation system, in which a part of high-temperature circulating feed water flowing in a circulation system is taken out therefrom to be introduced into a high-pressure feed water heater from the upstream side thereof and passed therethrough, whereby the feed water heater can be warmed during a period of time between an instant, at which the boiler is ignited, and an instant, at which the high-pressure feed water heater is put in practical use, and in which the occurrence of an excessively high thermal stress in the high-pressure feed water heater can be prevented.